Sensing of two-dimensional residual mass structure on a photoreceptor after transfer is used to identify specific types of transfer defects. Upon identification, closed-loop control of the transfer process can be performed taking into account the identified defect types, as well as their magnitudes, to correct or compensate for the defects.
The use of sensors to detect the toner mass levels on a photoreceptor, or other substrate, in a post-development position (detection of developed mass) in a xerographic engine is known. For example, see U.S. Pat. No. 5,887,221 to Grace; and U.S. Pat. No. 5,543,896 to Mestha; and U.S. Pat. No. 6,694,109 to Donaldson et al. The use of sensors to detect residual toner mass levels post-cleaning device is also known. For example, see U.S. Pat. No. 6,272,295 to Lindblad et al. and U.S. Pat. No. 5,903,797 to Daniels et al. It is also known to measure the residual mass after transfer but before the cleaning device (post transfer residual mass).
Previous post-transfer residual mass sensors have provided information about the average transfer efficiency and could enable limited closed loop control of the transfer system. For example, some teach use of an Extended Toner Area Coverage (ETAC) sensor to measure residual mass per unit area (RMA) during xerographic setup. The data from the sensor in this case is used to adjust the transfer shield current setpoint to obtain optimal performance prior to the submission of the customer's job.
The information provided by measuring the RMA with a point sensor like an ETAC is limited to an average measurement of transfer performance. In addition, because a point sensor typically only measures the transfer efficiency at one isolated location in the cross process direction, variations that occur across the belt are not captured by this type of sensor. Therefore, typical ETAC sensors provide only minimal information that is relevant to control of the transfer performance.
To overcome this problem, subsequent implementations have used sensors containing arrays of optical sensing elements. In many of these devices, the array of sensing elements provides information across the entire surface of the photoconductor or other substrate of interest. Such optical sensing array devices are termed full-width array (FWA) sensors. These FWA sensors have been used for measuring RMA across all or a majority of the photoreceptor surface. This method eliminated concerns of the point-sensing nature of ETAC RMA sensors because the residual mass content of the entire image area of the photoreceptor could now be captured. However, such prior methods were still only concerned with measuring average transfer efficiency. Thus, although the RMA value obtained may be more sensitive or accurate than prior point sensors because it averages over a larger area, such sensing systems are still not fully utilizing the information that is available from the FWA sensor.